Method and apparatus for monitoring heart rate

ABSTRACT

A monitoring device for implant in a patient&#39;s body. The device is provided with a physiologic sensor generating output signals, which signals are stored as numerical values. The device defines first monitoring periods limited to time periods during successive nights during which the patient is likely to be asleep, stores the numerical values generated during the first monitoring periods in a memory and calculates values reflecting general levels of the numerical values during the first monitoring periods. The device may also or alternatively define second monitoring periods limited to time periods during successive daytime and store numerical values generated during the second monitoring periods. The physiologic sensor may be an electrogram amplifier or other sensor indicative of metabolic demand for oxygenated blood, such as an activity sensor. The calculated values reflecting general levels of the numerical values generated during the monitoring periods may be average values, such as mean values or over-all total values, depending upon the sensor employed.

This application claims the benefit of Provisional application Ser. No.60/123,002, filed Mar. 5, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to implantable medical devicesand more particularly to implantable medical devices intended for use inmonitoring a patient's heart rhythm.

Implantable pacemakers and cardioverters monitor the heart's rhythm inorder to detect arrhythmias and deliver appropriate therapies toterminate detected arrhythmias. In conjunction with this function, theability of the device is to store information with regard to monitoredheart rhythms has dramatically increased over the past two years.Examples of implantable pacemakers and defibrillators which have thecapability of storing information related to monitor heart rhythmsinclude U.S. Pat. No. 4,223,678 issued to Langer et al., U.S. Pat. No.5,722,999 issued to Snell, U.S. Pat. No. 5,513,645 issued to Jacobsen etal. and U.S. Pat. No. 5,312,446 issued to Holschbach et al. In addition,there have recently been developed implantable monitoring devices thatdo not deliver any anti-arrhythmia therapies to the heart but simplystore information regarding a patient's heart rhythms for later uplinkto an external device. Such devices are disclosed in U.S. Pat. No.5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued toAdams and U.S. Pat. No. 5,497,780 issued to Zehender.

In conjunction with implantable devices as described above, informationstored relating to a patient's heart rhythm may include informationrelating to heart rate trends over time, as disclosed in U.S. Pat. No.5,088,488 issued to Markowitz et al., U.S. Pat. No. 5,330,513 issued toNichols et al. and U.S. Pat. No. 5,603,331 issued to Heemels et al. aswell as information relating to heart rate variability over time, asdisclosed in U.S. Pat. No. 5,749,900 issued to Schroeppel et al., U.S.Pat. No.5,466,245 issued to Spinelli et al., U.S. Pat. No. 5,411,131issued to Yomtov et al. and U.S. Pat. No. 5,437,285 issued to Verrier etal. Typically, measurements of heart rate trend in such devices areaccomplished by continually measuring heart rate over a defined timeperiod, and calculating average heart rates for successive shorter timeperiods within the defined time period for later telemetry to anexternal device. Gradual increases in average heart rate over extendedtime periods are known to be an indicator of decompensation, aphenomenon that takes place during the progression of clinical heartfailure.

SUMMARY OF THE INVENTION

The present invention is directed toward an implantable device havingenhanced capabilities for monitoring a patient's heart rate trends overextended periods of time. The information collected by the implantabledevice is stored and telemetered to an associated external device suchas a device programmer for display and analysis. Heart rates aremeasured by measuring the time intervals between sensed depolarizationsof a chamber of the patient's heart and preceding sensed depolarizationsor delivered pacing pulses. Intervals may be measured in the ventricleand/or atrium of the patient's heart. The measured intervals arereferred to hereafter as “heart intervals”. The measured heart intervalsduring defined time periods are used to calculate average heart rates oraverage heart intervals associated with the time periods. Preferably theaverage heart rate takes the form of a mean heart rate, but in someembodiments, the median heart rate over the time periods may be employedor the most common heart rate or interval based on a in a histogram ofmeasured heart intervals or other equivalent value may be substituted.For purposes of the present application, the term “average heart rate”should be understood to include mean, median or other equivalent valuesindicative of the general heart rate or heart interval.

Rather than simply measuring average heart rate values over successivetime periods, the implantable device instead measures successive averagevalues of heart rates measured during discontinuous time periods,preferably chosen to occur during times of particular interest, forexample during defined time periods during the night and/or day.Preferably the measurements are taken and stored over a period of weeksor months. In a first embodiment, measurements are during the nightduring a period of time in which the patient is likely to be sleeping.In this context, measurement of the trend of night heart rates taken,for example over the period of time between 12:00 a.m. and 4:00 a.m . isbelieved to be particularly valuable. Night heart rate is predominantlycontrolled by the parasympathetic nervous system, and progression ofheart failure is usually associated with abnormal excitation of thesympathetic nervous system, leading to increases in night heart rate.

In addition, long-term trends of daytime heart rates may also becollected, for example over periods of time between 8:00 a.m. and 8:00p.m. Daytime heart rate is primarily controlled by the sympatheticnervous system and thus differences in day and night heart rates can beused as a measure of autonomic dysfunction and have been shown to bedifferent in heart failure patients when compared to age matchedindividuals with normal hearts. In the context of an implantablepacemaker, comparisons of trends of day and night heart rates to thelower or base pacing rate of the pacemaker may also provide usefulphysiological information. This comparison may be especially valuable inpacemakers which store information regarding trends of physiologicsensor outputs or regarding trends of pacing rates based uponphysiologic sensor outputs as in U.S. patent application Ser. No.09/078,221, filed May 13, 1998 by Stone et al, incorporated herein byreference in its entirety.

In a preferred embodiment of the invention, the implantable deviceincludes a sensor indicative of exercise level either measured directlyusing a physiologic sensor such as an accelerometer or piezo-electricsensor or measured indirectly by means of a sensor of metabolic demandsuch as a pressure sensor, oxygen saturation sensor, stroke volumesensor or respiration sensor. In this embodiment of the invention,measurements of heart rhythms are made only in response to the sensor'sdetermination that the patient is at rest, in order to produce along-term trends of resting heart rates during the defined timeintervals. Even over relatively long time frames, a patient's level ofactivity may vary substantially, and changes in average heart rates canbe masked by such variations in exercise level. By limiting themeasurements of heart rates to times during which the patient is knownto be at rest, a more accurate indication of the true long-termprogression of heart rates can be obtained. In such embodiments theimplantable device may collect heart rate information continuouslyduring longer time periods, typically extending at least over severalhours. During the longer time periods the device may define a series ofshorter time periods, typically extending over several minutes, and willemploy heart rate information collected during a preceding one of theshorter time periods only if the sensor indicates the patient was atrest during the shorter time period.

In some preferred embodiments, particularly those intended for use inpatients known to suffer from tachyarrhythmias, the implantable deviceis also configured to reject intervals between depolarizationsassociated with tachyarrhythmias. In such embodiments the implantabledevice may define a minimum cumulative duration of non-rejected heartintervals as a prerequisite to calculation of an average rate value fora defined time period.

In devices employing physiologic sensors, the device may correspondinglyalso store values indicative of the general levels of sensor outputduring daytime and nighttime periods may also be collected. In suchembodiments, average sensor output values, including the various typesof averages discussed above in conjunction with calculation of averageheart rates may be employed. Alternatively, a sum or total of allgenerated sensor outputs during relevant time periods may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable pacemaker/cardioverter/ defibrillatorof a type useful in practicing the present invention, in conjunctionwith a human heart.

FIG. 2 illustrates an implantable pacemaker of a type useful inpracticing the present invention, in conjunction with a human heart.

FIG. 3 illustrates an implantable monitor of a type useful in practicingthe present invention.

FIG. 4 is a perspective view of a programmer of a type useful inpracticing the present invention.

FIG. 5 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator of a type useful in practicing thepresent invention.

FIG. 6 is a functional schematic diagram of an implantable pacemaker ofa type useful in practicing the present invention.

FIG. 7 is a functional schematic diagram of an implantable monitor of atype useful in practicing the present invention.

FIG. 8 is a functional schematic diagram of a programmer of a typeuseful in practicing the present invention.

FIG. 9 is a functional flow chart illustrating a first method ofmonitoring heart rate trends, which may be employed in conjunction withthe present invention.

FIG. 10 is a functional flow chart illustrating a second method ofmonitoring heart rate trends, which may be employed in conjunction withthe present invention.

FIG. 11 is a functional flow chart illustrating a method of monitoringsensor output trends, which may be employed in conjunction with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a defibrillator and lead set of a type in which thepresent invention may usefully be practiced. The ventricular leadincludes an elongated insulative lead body 16, carrying three mutuallyinsulated conductors. Located adjacent the distal end of the lead are aring electrode 24, an extendable helix electrode 26, mounted retractablywithin an insulative electrode head 28, and an elongated coil electrode20. Each of the electrodes is coupled to one of the conductors withinthe lead body 16. Electrodes 24 and 26 are employed for cardiac pacingand for sensing ventricular depolarizations. At the proximal end of thelead is a bifurcated connector 14 that carries three electricalconnectors, each coupled to one of the coiled conductors.

The atrial/SVC lead includes an elongated insulative lead body 15, alsocarrying three mutually insulated conductors. Located adjacent theJ-shaped distal end of the lead are a ring electrode 21 and anextendible helix electrode 17, mounted retractably within an insulativeelectrode head 19. Each of the electrodes is coupled to one of theconductors within the lead body 15. Electrodes 17 and 21 are employedfor atrial pacing and for sensing atrial depolarizations. An elongatedcoil electrode 23 is provided, proximal to electrode 21 and coupled tothe third conductor within the lead body 15. At the proximal end of thelead is a bifurcated connector 13 that carries three electricalconnectors, each coupled to one of the coiled conductors.

The coronary sinus lead includes an elongated insulative lead body 6,carrying one conductor, coupled to an elongated coiled defibrillationelectrode 8. Electrode 8, illustrated in broken outline, is locatedwithin the coronary sinus and great vein of the heart. At the proximalend of the lead is a connector plug 4, which carries an electricalconnector, coupled to the coiled conductor.

The pacemaker/cardioverter/defibrillator 10 includes a hermeticenclosure 11 containing the electronic circuitry used for generatingcardiac pacing pulses for delivering cardioversion and defibrillationshocks and for monitoring the patient's heart rhythm.Pacemaker/cardioverter/defibrillator 10 is shown with the lead connectorassemblies 4, 13 and 14 inserted into the connector block 12, whichserves as a receptacle and electrical connector for receiving theconnectors, 4, 13 and 14 and interconnecting the leads to the circuitrywithin enclosure 11. An activity sensor 30 is illustrated schematicallyby broken outline, and may be an accelerometer or a piezoelectrictransducer. Sensor 30 may be used for verifying that the patient is atrest, in conjunction with measurement of long-term heart rate trendsaccording to the present invention as well as for regulation of pacingrate based upon demand for cardiac output.

Optionally, insulation of the outward facing portion of the housing 11of the pacemaker/cardioverter/defibrillator 10 may be provided or theoutward facing portion may instead be left uninsulated, or some otherdivision between insulated and uninsulated portions may be employed. Theuninsulated portion of the housing 11 optionally serves as asubcutaneous defibrillation electrode, used to defibrillate either theatria or ventricles. Other lead configurations and electrode locationsmay of course be substituted for the lead set illustrated. For example,atrial defibrillation and sensing electrodes might be added to eitherthe coronary sinus lead or the right ventricular lead instead of beinglocated on a separate atrial lead, allowing for a two-lead system.

FIG. 2 illustrates a cardiac pacemaker of a type appropriate for use inpracticing the present invention in conjunction with its associated leadsystem, illustrated in relation to a patient's heart. The pacemaker 120includes a hermetic enclosure 124 containing the electronic circuitryused for generating cardiac pacing pulses and for monitoring thepatient's heart rhythm. An activity sensor 126 is illustratedschematically by broken outline, and may be an accelerometer or apiezoelectric transducer as discussed above in conjunction with FIG. 1.Mounted to the enclosure 124 is a header 122 which serves as areceptacle and electrical connector for receiving the connectors 132 and134 of pacing leads 128 and 130 and interconnecting the leads to thecircuitry within enclosure 124. Lead 128 is a ventricular lead providedwith electrodes 140 and 142 for monitoring right ventricular heartsignals. Also illustrated on lead 128 is a physiologic sensor 144 whichmay optionally be included in addition to or as an alternative to theactivity sensor 126, and which may take the form of an oxygen sensor,pressure sensor, temperature sensor, other sensor of any of the varioustypes employed for monitoring demand for cardiac output or for measuringheart hemodynamics. Sensor 144 may be used in conjunction with or as analternative to the activity sensor 126 for verifying that the patient isat rest, in conjunction with measurement of long-term heart rate trendsaccording to the present invention. Atrial lead 130 carries electrodes136 and 138 and is employed for sensing and pacing the patient's atrium.

FIG. 3 illustrates a subcutaneously implantable monitor of a typeappropriate for use in practicing the present invention. The monitorshares the external configuration of the Medtronic Reveal ® implantablemonitor, and is provided with a hermetically sealed enclosure 104containing the electronic circuitry used for generating cardiac pacingpulses and for monitoring the patient's heart rhythm and which carries amolded plastic header 108. The enclosure 104 and the header 108 eachcarry an electrode 102 and 106, respectively for monitoring heartrhythm. Also 30 mounted in the header 108 is an antenna 110 for use incommunicating between the device and an external programmer. Illustratedin broken outline at 112 is an internal activity sensor, of the typetypically employed in the context of rate responsive cardiac pacemakers,taking the form either of an accelerometer or a piezo-electrictransducer. Heart signals are detected between the electrodes 102 and106 and measurements of physical activity are detected by sensor 112 foruse in storing and calculating heart rate trends and heart ratevariability measurements according to the present invention.

FIG. 4 is a plan view of an external programmer of a sort appropriatefor use in conjunction with the practice of the present invention inconjunction with any of the devices of FIGS. 1-3. The programmer 420 isa microprocessor controlled device which is provided with a programminghead 422 for communicating with an implanted device, a set of surfaceelectrogram electrodes 459 for monitoring a patient's electrogram, adisplay 455 which is preferably a touch sensitive display, controlbuttons or keys 465, and a stylist 456 for use in conjunction with thetouch sensitive screen 455. By means of the control keys 465 and thetouch sensitive screen 455 and stylus 456, the physician may formatcommands for transmission to the implantable device. By means of thescreen 455, the physician may observe information telemetered from theimplantable device. The programmer is further provided with a printer463 which allows for hard copy records of displays of signals receivedfrom the implanted device such as electrograms, stored parameters,programmed parameters and information as to heart rate trends accordingto the present invention. While not visible in this view, the device mayalso be provided with a floppy disk or CD ROM drive and/or a port forinsertion of expansion cards such as P-ROM cartridges, to allow forsoftware upgrades and modifications to the programmer 420.

In the context of the present invention, programmer 420 may serve simplyas a display device, displaying information with regard to heart ratevariability and heart rate trends as calculated by the implanted deviceor instead may receive uplinked raw data related to heart intervals andmay calculate the heart rate trends and heart rate variability valuesaccording to the present invention. It is believed that it is preferablefor the implanted device to perform the bulk of the computationsnecessary to practice the invention, and in particular that it ispreferable for the implanted device to at least calculate average ratevalues, to reduce the storage requirements within the implanted device.However, allocation of functions between the implanted device and theprogrammer may differ from the preferred embodiments and still result ina workable system.

FIG. 5 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator of the type illustrated in FIG. 3,in which the present invention may usefully be practiced. This diagramshould be taken as exemplary of one type of anti-tachyarrhythmia devicein which the invention may be embodied, and not as limiting, as it isbelieved that the invention may usefully be practiced in a wide varietyof device implementations, including devices providing therapies fortreating atrial arrhythmias instead of or in addition to ventriculararrhythmias, cardioverters and defibrillators which do not provideanti-tachycardia pacing therapies, anti-tachycardia pacers which do notprovide cardioversion or defibrillation, and devices which deliverdifferent forms of anti-arrhythmia therapies such nerve stimulation ordrug administration.

The device is provided with a lead system including electrodes, whichmay be as illustrated in FIG. 1. Alternate lead systems may of course besubstituted. If the electrode configuration of FIG. 1 is employed, thecorrespondence to the illustrated electrodes is as follows. Electrode311 corresponds to electrode 11, and is the uninsulated portion of thehousing of the implantable pacemaker/cardioverter/defibrillator.Electrode 320 corresponds to electrode 20 and is a defibrillationelectrode located in the right ventricle. Electrode 310 corresponds toelectrode 8 and is a defibrillation electrode located in the coronarysinus. Electrode 318 corresponds to electrode 28 and is a defibrillationelectrode located in the superior vena cava. Electrodes 324 and 326correspond to electrodes 24 and 26, and are used for sensing and pacingin the ventricle. Electrodes 317 and 321 correspond to electrodes 19 and21 and are used for pacing and sensing in the atrium.

Electrodes 310, 311, 318 and 320 are coupled to high voltage outputcircuit 234. Electrodes 324 and 326 are coupled to the R-wave amplifier200, which preferably takes the form of an automatic gain controlledamplifier providing an adjustable sensing threshold as a function of themeasured R-wave amplitude. A signal is generated on R-out line 202whenever the signal sensed between electrodes 324 and 326 exceeds thepresent sensing threshold.

Electrodes 317 and 321 are coupled to the P-wave amplifier 204, whichpreferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredR-wave amplitude. A signal is generated on P-out line 206 whenever thesignal sensed between electrodes 317 and 321 exceeds the present sensingthreshold. The general operation of the R-wave and P-wave amplifiers 200and 204 may correspond to that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., issued Jun. 2, 1992, for an Apparatus for MonitoringElectrical Physiologic Signals, incorporated herein by reference in itsentirety. However, any of the numerous prior art sense amplifiersemployed in implantable cardiac pacemakers, defibrillators and monitorsmay also usefully be employed in conjunction with the present invention.

Switch matrix 208 is used to select which of the available electrodesare coupled to wide band amplifier 210 for use in digital signalanalysis. Selection of electrodes is controlled by the microprocessor224 via data/address bus 218, which selections may be varied as desired.Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228.Microprocessor 224 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 226 torecognize and classify the patient's heart rhythm employing any of thenumerous signal-processing methodologies known to the art.

Telemetry circuit 330 receives downlink telemetry from and sends uplinktelemetry to the patient activator by means of antenna 332. Data to beuplinked to the activator and control signals for the telemetry circuitare provided by microprocessor 224 via address/data bus 218. Receivedtelemetry is provided to microprocessor 224 via multiplexer 220. Theatrial and ventricular sense amp circuits 200, 204 produce atrial andventricular EGM signals, which also may be digitized, and uplinktelemetered to an associated programmer on receipt of a suitableinterrogation command. The device may also be capable of generatingso-called marker codes indicative of different cardiac events that itdetects. A pacemaker with marker-channel capability is described, forexample, in U.S. Pat. No.4,374,382 to Markowitz, which patent is herebyincorporated by reference herein in its entirety. The particulartelemetry system employed is not critical to practicing the invention,and any of the numerous types of telemetry systems known for use inimplantable devices may be used. In particular, the telemetry systems asdisclosed in U.S. Pat. No. 5,292,343 issued to Blanchette et al., U.S.Pat. No. 5,314,450, issued to Thompson, U.S. Pat. No.5,354,319, issuedto Wybomy et al. U.S. Pat. No. 5,383,909, issued to Keimel, U.S. Pat.No. 5,168,871, issued to Grevious, U.S. Pat. No. 5,107,833 issued toBarsness or U.S. Pat. No. 5,324,315, issued to Grevious, allincorporated herein by reference in their entireties, are suitable foruse in conjunction with the present invention. However, the telemetrysystems disclosed in the various other patents cited herein which aredirected to programmable implanted devices, or similar systems may alsobe substituted. The telemetry circuit 330 is of course also employed forcommunication to and from an external programmer, as is conventional inimplantable anti-arrhythmia devices.

The device of FIG. 5 may additionally is provided with an activitysensor 344, mounted to the interior surface of the device housing or tothe hybrid circuit within the device housing. The sensor 344 and sensorpresent in circuitry 342 may be employed in the conventional fashiondescribed in U.S. Pat. No. 4,428,378 issued to Anderson et al,incorporated herein by reference in its entirety, to regulate theunderlying pacing rate of the device in rate responsive pacing modes andalso serves as in an indicator of the patient's activity level for usein conjunction with the measurement of heart rate at rest or duringsleep, as discussed above and as discussed in more detail below inconjunction with FIGS. 10 and 12. In addition, the sensor 344 may beemployed to track the functional status of the patient as in theabove-cited application by Stone et al. In such case, the device mayalso store trend information with regard to the number of and/ordurations of periods in which the patient's physical activity meets orexceeds a defined level. Comparisons of the stored trend of day and/ornight heart rate with trend information related to sensor output may beespecially valuable.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies, and, for purposes ofthe present invention may correspond to circuitry known in the priorart. An exemplary apparatus is disclosed for accomplishing pacing,cardioversion and defibrillation functions as follows. The pacertiming/control circuitry 212 includes programmable digital counterswhich control the basic time intervals associated with DDD, VVI, DVI,VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes ofsingle and dual chamber pacing well known to the art. Circuitry 212 alsocontrols escape intervals associated with anti-tachyarrhythmia pacing inboth the atrium and the ventricle, employing, any anti-tachyarrhythmiapacing therapies known to the art.

Intervals defined by pacing circuitry 212 include atrial and ventricularpacing escape intervals, the refractory periods during which sensedP-waves and R-waves are ineffective to restart timing of the escapeintervals and the pulse widths of the pacing pulses. The durations ofthese intervals are determined by microprocessor 224, in response tostored data in memory 226 and are communicated to the pacing circuitry212 via address/data bus 218. Pacer circuitry 212 also determines theamplitude of the cardiac pacing pulses under control of microprocessor224.

During pacing, the escape interval counters within pacer timing/controlcircuitry 212 are reset upon sensing of R-waves and P-waves as indicatedby signals on lines 202 and 206, and in accordance with the selectedmode of pacing on time-out trigger generation of pacing pulses by paceroutput circuits 214 and 216, which are coupled to electrodes 317, 321,324 and 326. The escape interval counters are also reset on generationof pacing pulses, and thereby control the basic timing of cardiac pacingfunctions, including anti-tachyarrhythmia pacing.

The durations of the intervals defined by the escape interval timers aredetermined by microprocessor 224, via data/address bus 218. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R-Rintervals, P-P intervals, PR intervals and R-P intervals, whichmeasurements are stored in memory 226 and are used in conjunction withthe present invention to measure heart rate variability and heart ratetrends and in conjunction with tachyarrhythmia detection functions.

Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing/control circuitry 212corresponding to the occurrences of sensed P-waves and R-waves andcorresponding to the generation of cardiac pacing pulses. Theseinterrupts are provided via data/address bus 218. Any necessarymathematical calculations to be performed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 212 take place following such interrupts. Microprocessor 224includes associated ROM in which the stored program controlling itsoperation as described below resides. A portion of the memory 226 (FIG.2) may be configured as a plurality of recirculating buffers, capable ofholding series of measured intervals, which may be analyzed in responseto the occurrence of a pace or sense interrupt to determine whether thepatient's heart is presently exhibiting atrial or ventriculartachyarrhythmia.

The arrhythmia detection method of the present invention may include anyof the numerous available prior art tachyarrhythmia detectionalgorithms. One preferred embodiment may employ all or a subset of therule-based detection methods described in U.S. Pat. No. 5,545,186 issuedto Olson et al. or in U.S. Pat. No. 5,755,736 issued to Gillberg et al.,both incorporated herein by reference in their entireties. However, anyof the various other arrhythmia detection methodologies known to the artmight also Insert Walt's case

In the event that an atrial or ventricular tachyarrhythmia is detected,and an anti-tachyarrhythmia pacing regimen is desired, timing intervalsfor controlling generation of anti-tachyarrhythmia pacing therapies areloaded from microprocessor 224 into the pacer timing and controlcircuitry 212, to control the operation of the escape interval counterstherein and to define refractory periods during which detection ofR-waves and P-waves is ineffective to restart the escape intervalcounters.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 224 employs the escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 224 activates cardioversion/defibrillation controlcircuitry 230, which initiates charging of the high voltage capacitors246, 248 via charging circuit 236, under control of high voltagecharging control line 240. The voltage on the high voltage capacitors ismonitored via VCAP line 244, which is passed through multiplexer 220 andin response to reaching a predetermined value set by microprocessor 224,results in generation of a logic signal on Cap Full (CF) line 254,terminating charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse is controlled by pacertiming/control circuitry 212. Following delivery of the fibrillation ortachycardia therapy the microprocessor then returns the device tocardiac pacing and awaits the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization. In theillustrated device, delivery of the cardioversion or defibrillationpulses is accomplished by output circuit 234, under control of controlcircuitry 230 via control bus 238. Output circuit 234 determines whethera monophasic or biphasic pulse is delivered, whether the housing 311serves as cathode or anode and which electrodes are involved in deliveryof the pulse.

FIG. 6 is a functional schematic diagram of the pacemaker 120illustrated in FIG. 2. The pacemaker of FIGS. 2 and 6 is essentially aset of subcomponents of the implantablepacemaker/cardioverter/defibrillator illustrated in FIGS. 1 and 5. Likethe device of FIG. 5, the pacemaker is a microprocessor-controlleddevice with microprocessor 189 operating under control of programmingstored in Read Only Memory (ROM) 191. In the device as illustrated,electrodes 136 and 138, intended for location in the atrium of thepatient's heart are coupled to an atrial amplifier 181 which maycorrespond to atrial amplifier 204 in FIG. 5. Similarly, ventricularelectrodes 140 and 142 are coupled to ventricular amplifier 182, whichmay correspond to ventricular amplifier 200 in FIG. 5. The outputs ofatrial and ventricular amplifiers 181 and 182 are input into timing andcontrol circuitry 183 which conforms generally to the pacer timing andcontrol circuitry 212 of FIG. 5, and which measures intervals betweendetected depolarizations and controls intervals between delivered pacingpulses as well as generating interrupts via data/address 192 to awakemicroprocessor 189 in response to delivery of a pacing pulse or sensingof a cardiac depolarization. Intervals between depolarizations measuredby timing/control circuitry 183 are stored in Random Access Memory (RAM)190 until processed by microprocessor 189 to derive average heart ratevalues. Atrial and ventricular pacing pulses delivered according to oneor more of the standard pacing modes described in conjunction with FIG.5 are produced by atrial and ventricular pulse generator circuits 184and 185 which may correspond to pulse generator circuits 215 ad 216 inFIG. 5.

The sensor illustrated in FIG. 6 may correspond to either an activitysensor 126 as described in conjunction with FIG. 2 above, a respirationsensor, for example as disclosed in U.S. Pat. No. 5,562,711 issued toYerich et al or a hemodynamic sensor 140, as described in conjunctionwith FIG. 2. If the sensor is an activity sensor, then sensor-processingcircuitry 186 may correspond to sensor processing circuitry 342discussed in conjunction with FIG. 5. However, if the sensor is arespiration or hemodynamic sensor, the sensor processing circuitry wouldcorrespond to the sort of processing circuitry typically associated withrespiration or hemodynamic sensors. For purposes of the presentinvention, the hemodynamic sensor may be, for example, an oxygensaturation sensor in conjunction with associated processing circuitry asdescribed in U.S. Pat. No. 5,903,701 issued to Moore, a pressure ortemperature sensor and associated sensor processing circuitry asdescribed in U.S. Pat. No. 5,564,434 issued to Halperin et al. or animpedance sensor and associated sensor processing circuitry as describedin U.S. Pat. No. 5,824,029issued to Weijand et al., all incorporatedherein by reference in their entireties, or may correspond to othertypes of physiologic sensors, as may be appropriate. As discussed inmore detail below, in the context of the present invention, the sensor126, 140 is employed to determine when the patient is in a restingstate, for purposes of controlling the gathering and storage ofinformation related to long term heart rate trends. Telemetry circuitry187 in conjunction with antenna 188 serves to transmit information toand receive information from an external programmer precisely asdescribed above in conjunction with the device of FIG. 5, includinginformation related to stored median interval values and heart ratevariability measurements in RAM 190, as calculated by microprocessor189.

FIG. 7 illustrates the functional organization of the subcutaneouslyimplantable heart monitor 100 illustrated in FIG. 3. This deviceconsists essentially of a set of subcomponents of the more complexembodiment of the invention disclosed in FIG. 5, and includes a senseamplifier 152 coupled to electrodes 102 and 106, illustrated in FIG. 1.Sense amplifier 152 may correspond to sense amplifier 204 or 200 in FIG.5. Like the device of FIG. 5, the implantable monitor may be amicroprocessor control device operating under control microprocessor 156with its functionality controlled primarily by software stored in theread only memory associated therein. In this context, amplifier 152detects the occurrence of heart depolarizations, with timing/controlcircuitry 154 serving to measure the durations between the detectedheart depolarizations and to generate interrupts awakeningmicroprocessor 156 so that it may store, analyze and process thedetected intervals. Random Access Memory (RAM) 158 serves to storemeasured and calculated parameters including the calculated averageheart rate values for later telemetry to an external device. Like thedevice in FIG. 5, timing and control circuitry communicates with themicroprocessor and the remaining circuitry by means of the address/databus 168. Telemetry system 162 may correspond to telemetry system 330 inFIG. 5 and, via antenna 110 transmits and receives information from theexternal programmer, including transmitting information with regard tothe calculated median rate values and heart variability values stored inRAM 158. Sensor 112 may correspond to sensor 344 in FIG. 5 and it may bea physical activity sensor as discussed above. The output of sensor 112is passed through sensor processing circuitry 166 which may correspondto sensor processing circuitry 342 in FIG. 5.

FIG. 8 is a functional schematic of a programmer as illustrated in FIG.4 appropriate for use in conjunction with the invention. Programmer 420is a personal computer type, microprocessor-based device incorporating acentral processing unit 450, which may be, for example, an Intel 80386or 80486 or Pentium microprocessor or the like. A system bus 451interconnects CPU 450 with a hard disk drive 452 storing operationalprograms and data and with a graphics circuit 453 and an interfacecontroller module 454. A floppy disk drive 466 or a CD ROM drive is alsocoupled to bus 451 and is accessible via a disk insertion slot withinthe housing of the programmer 420. Programmer 420 further comprises aninterface module 457, which includes digital circuit 458, non-isolatedanalog circuit 459, and isolated analog circuit 460. Digital circuit 448enables interface module 457 to communicate with interface controllermodule 454.

In order for the physician or other caregiver or user to communicatewith the programmer 420, control buttons 465 or optionally a keyboardcoupled to CPU 50 are provided. However the primary communication modeis through graphics display screen 455 of the well-known “touchsensitive” type controlled by graphics circuit 453. A user of programmer420 may interact therewith through the use of a stylus 456, also coupledto graphics circuit 453, which is used to point to various locations onscreen 455, which display menu choices for selection by the user or analphanumeric keyboard for entering text or numbers and other symbols.

Graphics display 455 also displays a variety of screens of telemeteredout data or real time data including measurements of heart ratevariability and heart rate trends according to the present invention.Programmer 420 is also provided with a strip chart printer 463 or thelike coupled to interface controller module 454 so that a hard copy of apatient's ECG, EGM, marker channel or of graphics displayed on thedisplay 455 can be generated.

As will be appreciated by those of ordinary skill in the art, it isoften desirable to provide a means for programmer 20 to adapt its modeof operation depending upon the type or generation of implanted medicaldevice to be programmed. Accordingly, it may be desirable to have anexpansion cartridge containing EPROM's or the like for storing softwareprograms to control programmer 420 to operate in a particular mannercorresponding to a given type or generation of implantable medicaldevice. In addition, in accordance with the present invention, it isdesirable to provide the capability through the expansion cartridge orthrough the floppy disk drive 66 or CD ROM drive.

The non-isolated analog circuit 459 of interface module 457 is coupledto a programming head 422, which is used to establish the uplink anddownlink telemetry links between the pacemaker 410 and programmer 420 asdescribed above. Uplink telemetered EGM signals are received inprogramming head 422 and provided to non-isolated analog circuit 459.Non-isolated analog circuit 459, in turn, converts the digitized EGMsignals to analog EGM signals and presents these signals on output linesA EGM OUT and V EGM OUT. These output lines may then be applied to astrip-chart recorder 463 to provide a hard-copy printout of the A EGM orV EGM for viewing by the physician. Similarly, the markers received byprogramming head 422 are presented on the MARKER CHANNEL output linefrom non-isolated analog circuit 459.

Isolated analog circuit 460 in interface module 547 is provided toreceive external ECG and electrophysiologic (EP) stimulation pulsesignals. In particular, analog circuit 460 receives ECG signals frompatient skin electrodes 459 and processes these signals before providingthem to the remainder of the programmer system in a manner well known inthe art. Circuit 460 further operates to receive the EP stimulationpulses from an external EP stimulator for the purposes of non-invasiveEP studies, as is also known in the art.

In order to ensure proper positioning of programming head 422 over theantenna of the associated implanted device, feedback is provided to thephysician that the programming head 422 is in satisfactory communicationwith and is receiving sufficiently strong RF signals. This feedback maybe provided, for example, by means of a head position indicator, e.g. alight-emitting diode (LED) or the like that is lighted to indicate astable telemetry channel.

FIG. 9 illustrates a functional flow chart describing a first method ofcalculation of average heart rates taken during desired time ranges overthe course of a day. For example, calculation of a daily heart rateaverage and a night heart rate average, to be employed in constructingday heart rate trends and night heart rate trends, for display on theassociated external programmer. In this context, the flow chart of FIG.9 starts from the assumption that the implanted device will collect themeasured heart intervals and calculate and store the average heartinterval values for day heart rate and/or night heart rate, with thecalculated average day heart rate and night simply displayed on theexternal device associated with the implanted device. In this context,it should also be understood that all calculations and processing of themeasured heart intervals is performed by the microprocessor within theimplanted device. However, as noted above, alternate divisions of tasksbetween the implanted and external devices are still believed to bewithin the scope of the invention.

At 600, the device is initialized and thereafter sets SUMNN=0 at 602.SUMNN is a running sum of the total duration of measured heart intervalsretained for use in calculation of average heart rate according to thepresent invention. The device also sets the value of NN=0 in 602. NN isthe running total of measured heart intervals employed in calculation ofaverage day or night heart rates according to the present invention. Thedevice then waits until the time of day falls within the desired timewindow extending from a start time “A” to an end time “B”. In thecontext of monitoring of average daily heart rate, the defined timerange may extend between 8:00 a.m. and 8:00 p.m., for example. In thecontext of a device which measures average nightly heart rate, thedefined range may extend between 12:00 a.m. and 4:00 a.m., for example.It should be also understood that the same device may make and storemeasurements of both average day heart rate and average night heartrate.

If the device determines that present time T is within the defineddesired time range for heart range monitoring, in response to a sensedor paced depolarization at 606, the device at 608 stores the measuredheart interval separating the sensed or paced depolarization 606 fromthe preceding paced or sensed depolarization, as measured inmilliseconds. At 610, the device determines whether the measured heartinterval is acceptable to be retained for use in measuring average heartrate or should be rejected. The desirability of rejecting measured heartintervals will depend upon the condition of the patient and the type ofdevice implanted. For example, in the case of a patient who is subjectto atrial or ventricular tachycardia, wherein the device employing thepresent invention is an implantablepacemaker/cardioverter/defibrillator, it may be desirable to discard allmeasured heart intervals associated with detection and treatment oftachyarrhythmias. For example the device may reject all intervals whichmeet tachyarrhythmia detection criteria due to their relatively shortduration, all intervals obtained during charging of the outputcapacitors of such a device prior to delivery of a cardioversion ordefibrillation shock and all intervals sensed during delivery ofanti-tachyarrhythrnia therapies such as anti-tachycardia pacing,cardioversion and defibrillation. In contrast, if the invention isembodied in a simple VVI-type pacemaker, and the patient is not subjectto tachyarrhythmias, there may be no need to discard any heart intervalsending on a sensed depolarization. In addition or as an alternative, inwhich the invention is embodied to include a dual chamber pacemakercapable of switching between various pacing modes in response todetected atrial tachyarrhythmias, it may be desirable to discard heartintervals measured during operation of the mode switch between pacingmodes.

If the measured heart interval is not rejected, the value of theinterval is added to SUMNN at 612, and the value of NN is incremented byone at 614. The device continues to increment the values of SUMNN and NNaccording to this mechanism until the present time T equals or exceedsthe defined expiration time B for heart rate monitoring. At 616, thedevice compares the total duration of measured and saved intervals to adesired total duration “X” which may reflect a predetermined proportionof the duration of the monitoring interval. For example, the value ofSUMNN may have to exceed 20% of the defined monitoring period. In theevent that the value of SUMNN is inadequate, the device stores anindication that no heart rate has been calculated for the monitoringperiod presently in effect at 620, and the device resets the values ofSUMNN and NN to zero at 602, awaiting the next defined monitoringinterval. If the value of SUMNN is adequate, the average heart rate HRis calculated by means of the equation HR=60,000/(SUMNN/NN) at 622, andthe value of HR, representing the average heart rate over the monitoringperiod is stored at 624 for later telemetry to the associated externaldevice and for display by the associated external device. The method ofoperation illustrated in FIG. 9 may be employed to collect and calculateaverage daily rates, average night heart rates, or both, for display onthe associated external device.

FIG. 10 illustrates an alternative embodiment of the present inventionin which an associated activity sensor or other metabolic sensor isemployed in order to assure that during the defined heart ratemonitoring periods, only heart intervals indicative of the patient atrest are employed in calculating average heart rates. It should be notedthat the method of operation illustrated in FIG. 10 also permits thecalculation of average resting heart rates over 24 hour periods, bysimply designating the desired monitoring period it is to be successive24 hour periods, as opposed to discreet periods within each 24 hourperiod.

After initialization at 700, the device sets SUMNN and NN to zero at702, as discussed above in conjunction with FIG. 9, and awaits thebeginning of the defined monitoring period at 704. At 706, the deviceinitiates the relatively shorter time period T1, over which thepatient's physical activity or other metabolic indicator of demand forcardiac output is to be monitored. The values of INTCOUNT, indicative ofthe number of intervals counted during this shorter time interval T1 andINTSUM, reflective of the total duration of intervals stored duringinterval T1 are reset to zero at 706. The value of T1 is preferablyfairly short, for example, in the range of a few minutes, for example,about two to five minutes. Thereafter, until expiration of the shorterperiod T1 at 712, each time a paced or sensed depolarization is occursat 708, the heart interval separating the depolarization from thepreceding depolarization is stored at 710, and the device determineswhether the stored interval should be rejected at 726, in a fashionanalogous to that described in conjunction with FIG. 9 above. If theinterval is saved, the value of INTCOUNT is incremented by one at 728and the value of INTSUM is incremented by the duration of the storedheart interval at 730. This process continues until expiration of timeperiod T1 at 712. Following expiration of T1 at 712, the device checksthe output of the sensor over the preceding time period T1 and comparesthe output to a defined threshold to determine whether the patient is atrest. For example, if the sensor output takes the form of successivenumerical values (e.g. counts) generated over T1, the sum, mean, ormedian of the numerical values generated during T1 may be calculated andanalyzed, for example by comparison to a threshold value, to determinewhether the patient was at rest during T1. If the sensor's output basedon directly measured activity or other measured metabolic demandindicator indicates the patient was not at rest, the intervals collectedduring the preceding shorter T1 period are discarded, and the next T1period is initiated at 706. If the activity sensor or other indicator ofmetabolic demand indicates that the patient was at rest during thepreceding shorter time period T1, the value of NN is incremented by thevalue of INTCOUNT at 732 and the value of SUMNN is incremented by INTSUMat 734. This process continues until the device determines at 736 thatthe present time T is equal to or after the expiration point B of thedefined monitoring period.

On expiration of the defined monitoring period, the device checks at 724to determine whether the value of SUMNN exceeds a desired totalduration, precisely as described above, in conjunction with FIG. 9. Ifthe total duration of stored heart intervals is less than the desiredtotal, the device stores an indication that no measurement of averageheart rate was stored for the monitoring period at 720. However, if thetotal duration of measured heart intervals is sufficiently great, thevalue of the average heart rate is calculated at 718 in the same fashionas discussed in conjunction with FIG. 10 above, and the stored value ofthe average heart rate for the monitoring interval is stored at 716 forlater telemetry to an associated external device for display thereon.

FIG. 11 illustrates a functional flow chart describing an alternativeembodiment of the present invention in which sensor outputs aremonitored over daytime or nighttime periods, in a manner analogous tothe collection of heart rate information as discussed in conjunctionwith FIGS. 9 and 10 above. The term “average” in the context of FIG. 11is the same as discussed above in conjunction with monitoring of heartrates. The sensor may be an activity sensor as described above or any ofthe various known physiologic sensors available for implant in the humanbody, including but not limited to sensors of metabolic demand foroxygenated blood, including oxygen saturation sensors, blood pressuresensors, blood temperature sensors, Ph sensors, respiration sensors andthe like, as discussed above.

Calculation of a daily sensor output and a night sensor output value maybe used, for example, in constructing day sensor output trends and nightsensor output trends for display on the associated external programmer.In this context, the flow chart of FIG. 11 starts from the assumptionthat the implanted device will collect the measured sensor output valuesand calculate and store average or total values for day sensor outputand/or night sensor output, with the calculated average or total valuedisplayed on the external device associated with the implanted device.In this context, it should also be understood that all calculations andprocessing of the measured sensor output values are performed by themicroprocessor within the implanted device. However, as noted above,alternate divisions of tasks between the implanted and external devicesare still believed to be within the scope of the invention.

At 800, the device is initialized and thereafter sets SUMSENS=0 at 602.SUMSENS is a running sum of the total of measured sensor outputsretained for use in calculation of average or total sensor outputaccording to the present invention. The device then waits until the timeof day falls within the desired time window extending from a start time“A” to an end time “B”. In the context of monitoring of daily sensoroutput, the defined time range may extend between 8:00 a.m. and 8:00p.m., for example. In the context of a device that measures nightlysensor output, the defined range may extend between 12:00 a.m. and 4:00a.m., for example. It should be also understood that the same device maymake and store measurements of both day and night sensor outputs.

If the device determines that present time T is within the defineddesired time range for heart range monitoring, in response to a newoutput sensor value at 806, the device at 808 stores the measured sensoroutput as a numerical value. The value of the sensor output (SO) isadded to SUMSENS at 812. The device continues to increment the values ofSUMSENS according to this mechanism until the present time T equals orexceeds the defined expiration time B for sensor output monitoring at816. On expiration of the defined time for sensor output monitoring, thedevice either stores SUMSENS at 820 or optionally calculates and storesan average sensor output value at 822 and 824, for example calculatedbased on SUMSENS and the duration of the defined time for sensor outputmonitoring or based on SUMSENS and the total number of sensor outputsincluded in SUMSENS, in a fashion analogous to that employed tocalculate heart rate averages according to the method illustrated inFIG. 9.

In conjunction with the above disclosure, we claim:
 1. A monitoringdevice for implant in a patient's body, comprising: a cardiacelectrogram amplifier; a sensing electrogram amplifier; timing means fordefining first monitoring periods limited to time periods duringsuccessive nights during which the patient is likely to be sleep; memorymeans for storing heart intervals between depolarizations of thepatient's heart sensed by the amplifier during the first monitoringperiods; and means for calculating average heart rates during the firstmonitoring periods.
 2. A device according to claim 1 wherein the timingmeans further comprises means for defining second monitoring periodslimited to time periods during successive daytime periods and whereinthe memory means further comprises means for storing heart intervalsbetween depolarizations of the patient's heart sensed by the amplifierduring the second monitoring periods and wherein the calculating meansfurther comprises means for calculating average heart rates during thesecond monitoring periods.
 3. A device according to claim 1, furthercomprising a sensor responsive to the patients level of exercise andwherein the calculating means comprises means responsive to the sensorfor calculating average heart rates based only on heart intervals storedwhile the patient is at rest.
 4. A device according to claim 1, furthercomprising means for rejecting heart intervals occurring duringtachyarrhythmias and wherein the calculating means comprises means forcalculating average heart rates based on heart intervals not rejected.5. A device according to claim 4 wherein the calculating means comprisesmeans for determining total durations of non-rejected heart intervalsduring the first monitoring periods and means for preventing calculationof an average heart rate if the total duration of non-rejected heartintervals during a first monitoring period is less than a requiredduration.
 6. A monitoring device for implant in a patient's body,comprising: a sensor responsive to the patient's level of exercise; acardiac electrogram amplifier; a sensing electrode coupled to an inputof the amplifier; means for defining first monitoring periods limited totime periods during which the sensor indicates that the patient islikely to be at rest; memory means for storing heart intervals betweendepolarizations of the patient's heart sensed by the amplifier duringthe first monitoring periods.
 7. A device according to claim 6, furthercomprising means for rejecting heart intervals occurring duringtachyarrhythmias and wherein the calculating means comprises means forcalculating average heart rates based on heart intervals not rejected.8. A device according to claim 7 wherein the calculating means comprisesmeans for determining total durations of non-rejected heart intervalsduring the first monitoring periods and means for preventing calculationof an average heart rate if the total duration of non-rejected heartintervals during a first monitoring period is less than a requiredduration.
 9. A monitoring device for implant in a patient's body,comprising: a cardiac electrogram amplifier; a sensing electrode coupledto an input of the amplifier; means for defining first monitoringperiods limited to successive daytime periods; memory means for storingheart intervals between depolarizatons of the patient's heart sensed bythe amplifier during the first monitoring periods; and means forcalculating average heart rates during the first monitoring periods. 10.A device according to claim 9, further comprising a sensor responsive tothe patients level of exercise and wherein the calculating meanscomprises means responsive to the sensor for calculating average heartrates based only on heart intervals stored while the patient is at rest.11. A device according to claim 9, further comprising means forrejecting heart intervals occurring during tachyarrhythmias and whereinthe calculating means comprises means for calculating average heartrates based on heart intervals not rejected.
 12. A device according toclaim 11 wherein the calculating means comprises means for determiningtotal durations of non-rejected heart intervals during the firstmonitoring periods and means for preventing calculation of an averageheart rate if the total duration of non-rejected heart intervals duringa first monitoring period is less than a required duration.
 13. Amonitoring device for implant in a patients body, comprising: aphysiologic sensor generating an output signal; means for generatingnumerical values indicative of sensor output signals; timing means fordefining first monitoring periods limited to time periods duringsuccessive nights during which the patient is likely to be asleep;memory means for storing the numerical values generated during the firstmonitoring periods; and means for calculating values reflecting generallevels of the numerical values during the first monitoring periods. 14.A device according to claim 13 wherein the timing means furthercomprises means for defining second monitoring periods limited to timeperiods during successive daytime periods and wherein the memory meansfurther comprises means for storing numerical values generated duringthe second monitoring periods and wherein the calculating means furthercomprises means for calculating values reflecting general levels of thenumerical values during the second monitoring periods.
 15. A deviceaccording to claim, 13 wherein the physiologic sensor is an electrogramamplifier.
 16. A device according to claim 13 wherein the physiologicsensor is a sensor indicative of metabolic demand for oxygenated blood.17. A device according to claim 16 wherein the physiologic sensor is anactivity sensor.
 18. A device according to claim 13 wherein thecalculated values reflecting general levels of the numerical values areaverages.
 19. A device according to claim 13 wherein the calculatedvalues reflecting general levels of the numerical values are totals.